Semiconductor lasers and solid state lasers have material-specific emission wavelengths and thus expansion of their wavelength ranges is directly connected to expansion of the fields of applications. While infrared sources have been used in environmental measurements and medical and biotechnology fields, their applications to automobile exhaust detection, laser ionization mass spectrometry, fruit sugar analysis, dental treatment, noninvasive blood test, and cerebral blood flow test are presently being studied.
However, light sources such as ruby lasers, yttrium-aluminum-garnet (YAG) lasers, and carbon dioxide lasers can emit only light of specific wavelengths. Although the wavelength of other light sources such as titanium-sapphire lasers is tunable, such light sources can only emit light having a wavelength near 650 nm to 1100 nm. Thus, it is not possible to obtain laser beams in all of the wavelength regions. Accordingly, wavelength conversion elements that can convert light having a specific wavelength emitted from a laser beam source into light having a different wavelength are desired.
Conventionally, wavelength conversion elements that use borate-based crystals such as barium borate (BBO) and lithium borate (LBO) have been known. According to such wavelength conversion elements, wavelength conversion is conducted by phase matching using the birefringence of the crystals. However, it is difficult to achieve a sufficient wavelength conversion efficiency using a wavelength conversion element that uses the birefringence of the crystals. Moreover, since the birefringence of the crystals is intrinsic to the crystals and cannot be adjusted, a wavelength conversion element that uses birefringence has low flexibility in terms of choice of wavelength, etc.
Wavelength conversion elements that use ferroelectric oxide crystals such as lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) have also been known. These ferroelectric oxide crystals have a bias (polarization) in a specific direction of the atomic arrangement, and a positive polarization and a negative polarization respectively occur at two ends due to this bias. The polarization can be partly reversed by applying an electric field. Accordingly, when a periodic domain-inversed structure is formed in the ferroelectric oxide crystals, the interactive length can be increased compared to when birefringence matching of borate crystals is used, thereby enabling highly efficient wavelength conversion.
Japanese Unexamined Patent Application Publication No. 2008-170710 (Patent Literature 1) discloses a wavelength conversion element that uses a compound semiconductor crystal that contains nitrogen (N) and at least one of gallium (Ga), aluminum (Al), and indium (In) and has a spontaneous polarization. In Patent Literature 1, a polarized structure having a spontaneous polarization periodically reversed into a two-dimensional lattice geometry is formed in the compound semiconductor crystal, and this polarized structure satisfies quasi phase matching (QPM) conditions two-dimensionally with respect to incoming light of a first wavelength. Accordingly, since the interactive length can be increased compared to when birefringence matching of borate crystals is used, highly efficient wavelength conversion is made possible.
Patent Literature 1 discloses a method for making a wavelength conversion element by forming a two-dimensional domain-inverted structure using a compound semiconductor crystal. In particular, a mask pattern corresponding to a two-dimensional domain-inverted structure pattern is formed on a gallium nitride (GaN) substrate having a +c face. Then a GaN layer is formed in the +c axis direction on the +c face of the GaN substrate and the mask pattern. In this case, a +c region is epitaxially grown on the +c face of the GaN substrate so that the thickness of the GaN layer increases in the +c axis direction, and a −c region is epitaxially grown on the mask layer so that the thickness of the GaN layer increases in the −c axis direction. Thus, a two-dimensional domain-inverted structure is formed.